Signal transduction through the B immunoglobulin receptor is now known to involve interconnecting enzyme pathways, catalyzed by tyrosine kinase and phospholipase C. The first enzyme system utilizes 3 src family kinase, Blk, Fyn, and Lyn, to phosphorylate a number of protein on tyrosine residues following receptor crosslinking. The substrates for these kinases have been investigated in B lymphocytes from normal and in Xid mice. One of the substrates has been tentatively identified as the proto- oncogene Vav. Vav coprecipitates with all of the src kinases found in B lymphocytes and has an intrinsic kinase activity associated with it. Another substrate that has been examined is a 72 Kd protein that is phosphorylated when the cells are simulated with anti-IgD but not ant-IgM, this protein is maximally phosphorylated by approx. 20 seconds and then dephosphorylated by 1 minute. This substrate has not been identified yet, but, as only 1% of phosphorylated proteins are phosphorylated on tyrosine and as the 72 Kd protein is rapidly dephosphorylated, the data imply that this protein is potentially a regulatory protein. The total phosphoproteins that are seen in simulated B lymphocytes from either normal or Xid mice are being examined by 2D electrophoresis in an attempt to fine a protein(s) that behaves differently from the 2 strains.